However bad life is for you in the Watch (especially the Old Watch during the rule of Lord Winder), you Do Not Drop Your Mates In The Cacky. Even during a storm of cack that is flying about, even under duress from a new senior Sergeant you Do Not Drop Your Mates In The Cacky. That would be akin to suicide.
Corporal Quirke is one of those who disregard this rule in order to throw said sergeant off the scent. The shock of this reverberates around the men, and he has no friends at all after this. When he's the one in the cacky, no-one helps him out.
In Roundworld English idiom, Cack is a slang term for faeces; nonsense or rubbish: "what a load of cack" could equally be used to describe someone talking nonsense or as a criticism of something of poor quality. Derived from an ancient Indo-European word, kakkos, cognate with German word Kacke, and Welsh word "cach".